1. Field of the Invention
The present invention generally relates to an exhaust emission purifying apparatus for an engine (hereinafter, referred to as an exhaust emission purifying apparatus), which is provided for reductively purifying nitrogen oxides (NOx) in an exhaust emission of the engine and, more particularly, relates to a technology for determining with high accuracy whether or not a supply system of a liquid reducing agent or precursor thereof brings about clogging.
2. Description of the Related Art
As a catalytic purification system which purifies NOx contained in the exhaust emission of an engine, there has been proposed, in Japanese Laid-open (Kokai) Patent Application Publication No. 2000-27627, an exhaust emission purifying apparatus. In this exhaust emission purifying apparatus, a liquid reducing agent or precursor thereof (to be referred to as “liquid reducing agent”) according to engine operating conditions is injection-supplied to the exhaust emission at a position upstream of a NOx reduction catalytic converter disposed in an exhaust emission passageway of an engine exhaust system, so that NOx in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction by which NOx is purified into harmless components.
Further, in this exhaust emission purifying apparatus, if a supply system of the liquid reducing agent (to be referred to as a reducing agent supply system) is clogged due to the deposition of a reducing agent component, the mixing-in of foreign substances or the like, the reducing agent of appropriate amount is not supplied to the NOx reduction catalytic converter, so that required NOx purification performance cannot be achieved. Therefore, there has been proposed, as disclosed in Japanese Laid-open (Kokai) Patent Application Publication No. 2006-77765, a technology for determining whether or not the reducing agent supply system is clogged based on a pressure reduction range, for a predetermined time, of the liquid reducing agent remaining in the reducing agent supply system, in a state where a pump for supplying the liquid reducing agent under pressure is stopped.
Since NOx is generated under a high temperature and high pressure condition, there is a state where NOx concentration in the exhaust emission is kept low depending on the engine operating conditions and accordingly, an exhaust emission purifying process does not need to be performed. Therefore, in the exhaust emission purifying apparatus using the reducing agent, from the standpoint of the prevention of unnecessary consumption of the reducing agent, such a configuration is employed that the liquid reducing agent is injection-supplied intermittently. Further, in the exhaust emission purifying apparatus, since the NOx concentration in the exhaust emission is changed along with the engine operating conditions, an injection amount of the liquid reducing agent is dynamically increased or decreased. Accordingly, the pressure reduction range for the predetermined time becomes very small and therefore, in the technology disclosed in Japanese Laid-open (Kokai) Patent Application Publication No. 2006-77765, there may be an erroneous determination of clogging although the reducing agent supply system has not yet brought about clogging.